[Help] Obtain oauth2 access token from Gateway Lambda Target

## Summary
Is there a way to use AgentCore identity to obtain an access token from a lambda function on behalf of a user who invoked AgentCore Gateway?

## Details
We have an an MCP client connecting to AgentCore Gateway, using Okta for inbound auth.

Gateway is invoking a lambda target, which means we must use IAM auth for outbound auth from the Gateway.

However our Lambda function requires access to downstream resources, like RDS / Redshift / S3.

We want to get an access token which on bahalf of the invoking user, to ensure they consent and so that RBAC configured at the resource is respected.

Is there a way to do this? Looking at the `client_context.custom` and `event` objects that are sent to the lambda there is nothing that identifies the user invoking the gateway.

## Expected Behavior
We could use AgentCore Identity to perform a 3LO flow from a lambda invoked from AgentCore Gateway.